Pre-Landfall 7 | Summary
Posting Date: August 28, 2012, 9:00:00 AM
Despite repeated forecasts of Isaac’s imminent intensification to hurricane status, Tropical Storm Isaac remained a tropical storm as of today’s 11:00 a.m. EDT advisory from the National Hurricane Center (NHC). Isaac is a large tropical cyclone with maximum sustained winds of about 70 mph, just shy of the 74 mph that would make it a hurricane. It is located about 105 miles south-southeast of the mouth of the Mississippi River—a region that could benefit from the rain, as the prolonged hydrological drought this year has caused the river to drop to historic lows and ground cargo ships. Barge traffic has been suspended along southern portions of the river, and the ports of Mobile, Alabama, and New Orleans have been closed. Isaac is also about 185 miles to the south-southeast of Biloxi, Mississippi, and is moving slowly northwestward at about 10 mph.
The NHC still expects Isaac to reach hurricane status today. Favorable wind shear conditions and warm sea surface temperature conditions, as reported by
AIR’s ClimateCast Atlantic Hurricane Conditions®, will facilitate this process. Landfall is currently forecast for somewhere along the Louisiana or Mississippi coast in the early hours of tomorrow.
Louisiana’s Governor Bobby Jindal has requested and been granted a federal disaster declaration. Residents from Louisiana to Mississippi are preparing for Isaac’s arrival by stocking up on food and water; New Orleans residents have been fleeing to higher ground, causing heavy traffic out of the city. Several shelters have been set up in and around the city.
Offshore Impacts
Offshore oil and gas operators in the Gulf of Mexico are evacuating platforms and rigs in the path of Tropical Storm Isaac. According to a news release issued yesterday by the federal Bureau of Safety and Environmental Enforcement (BSEE), personnel have been evacuated from 346 production platforms, equivalent to 58% of the 596 manned platforms in the Gulf of Mexico. Approximately 78% of the current daily oil production and approximately 48% of the daily natural gas production in the Gulf of Mexico has been shut-in.
Exposure at Risk
AIR estimates the total insured
exposure in coastal counties within Isaac’s expected impact area (within the NHC cone of uncertainty) totals USD 480 billion, or roughly 21% of the combined insured exposure in Louisiana, Mississippi, and Alabama.
The majority of single-family residential structures along the U.S. Gulf Coast are of masonry construction. Since there is a possibility of Category 1 or weak Category 2 wind speeds, these structures can experience moderate damage to the roof, roof covering, with little damage to masonry walls expected. Failure of roofing components often occurs because of improper fastening between the roof system and building frame. At Category 1 wind speeds, there may also be numerous cases of damage due to downed trees, exacerbated by saturated soils, or from loose building debris such as shingles or roofing tiles.
It should be noted that the vulnerability of mobile (manufactured) homes and light metal structures is much greater than that of other construction types; these buildings could experience structural damage. Mobile home exposure accounts for 2% of the overall exposure in the impacted states. Engineered structures, such as reinforced concrete and steel buildings, should experience less damage compared to residential wood-frame and masonry structures; they may exhibit isolated instances of nonstructural damage, such as that to windows and roof coverings. However, damage to contents and internal finishes could result from nonstructural damage.
In addition, it is important to note that building vulnerability can change significantly through changes in building code and code enforcement, changes in material and construction practices, and structural aging. Similarly, the awareness surrounding building practices in Louisiana, Mississippi, and Alabama increased after Hurricane Katrina. Louisiana adopted statewide building codes in 2007, as did five coastal counties in Mississippi (2006) and Mobile and Baldwin Counties in Alabama (2001 and 2007, respectively). Inland counties in Mississippi and Alabama are currently not covered by any statewide structural design regulations. AIR expects that newer structures in the regions with updated building codes and improved levels of enforcement will perform better than older structures.
Forecast Track and Intensity
Isaac’s sustained winds have increased slightly to 70 mph and the storm is expected to achieve hurricane status later today. Data from Hurricane Hunter aircraft indicate that maximum winds are closer to the storm’s center, suggesting that the strengthening of Isaac is imminent. These data also indicate that the storm has wobbled on a generally westward course during the past few hours, which is expected to be a temporary deviation from its northwestward movement. Isaac is expected to move into a weakness in the subtropical ridge near the north-central Gulf Coast and slow its forward speed even more during the next couple of days as a ridge builds slightly to the northwest of it. Later, it should turn northward and northwestward into the Ohio Valley region. The speed has slowed, which may hold Isaac over water for an extra six to 12 hours longer than previously thought. Thus the center of the storm should come on shore about 8:00 a.m. tomorrow and push inland over New Orleans during the day. Given this track and the slow forward speed, surge into Lake Pontchartrain is still a threat, even if Isaac remains only a marginal hurricane.
Dangerous storm surges of up to 12 feet, heavy rainfall of up to 20 inches in areas, and strong winds are expected to extend well away from the center and to affect a large portion of the northern Gulf Coast. Heavy rainfall and flooding are also expected to spread inland over the Lower Mississippi Valley region during the next few days.
NHC’s Tropical Storm Warning has been extended westward along the northern Gulf Coast to just east of High Island, Texas; along the northern Gulf Coast to Cameron, Louisiana; and is in effect for the Alabama-Florida border to the Aucilla River. A Hurricane Warning is in effect for east of Morgan City, Louisiana, to the Alabama-Florida border, including metropolitan New Orleans, Lake Pontchatrain, and Lake Maurepas.